There is an ongoing effort to develop systems that are more energy-efficient. A large proportion (some estimates are as high as twenty-five percent) of the electricity generated in the United States each year goes to lighting, a large portion of which is general illumination (e.g., downlights, flood lights, spotlights and other general residential or commercial illumination products).
Solid state light emitters (e.g., light emitting diodes) are receiving much attention due to their energy efficiency. It is well known that incandescent light bulbs are very energy-inefficient light sources—about ninety percent of the electricity they consume is released as heat rather than light. Fluorescent light bulbs are more efficient than incandescent light bulbs (by a factor of about 10) but are still less efficient than solid state light emitters, such as light emitting diodes.
In addition, as compared to the normal lifetimes of solid state light emitters, incandescent light bulbs have relatively short lifetimes, i.e., typically about 750-1000 hours. Fluorescent bulbs have longer lifetimes (e.g., 10,000-20,000 hours) than incandescent lights, but provide less favorable color reproduction. In comparison, light emitting diodes, for example, have typical lifetimes between 50,000 and 70,000 hours.
General illumination devices are typically rated in terms of their color reproduction, i.e., the extent to which objects illuminated by the illumination devices are perceived to be the color that they actually are. Color reproduction is typically measured using the Color Rendering Index (CRI Ra). CRI Ra is a modified average of the relative measurements of how the color rendition of an illumination system compares to that of a reference radiator when illuminating eight reference colors, i.e., it is a relative measure of the shift in surface color of an object when lit by a particular lamp. The CRI Ra equals 100 if the color coordinates of a set of test colors being illuminated by the illumination system are the same as the coordinates of the same test colors being irradiated by the reference radiator. Daylight has a high CRI (Ra of approximately 100), with incandescent bulbs also being relatively close (Ra greater than 95), and fluorescent lighting being less accurate (typical Ra of 70-80). Certain types of specialized lighting have very low CRI (e.g., mercury vapor or sodium lamps have Ra as low as about 40 or even lower).
Another issue faced by conventional light fixtures is the need to periodically replace the lighting devices (e.g., light bulbs, etc.). Such issues are particularly pronounced where access is difficult (e.g., vaulted ceilings, bridges, high buildings, traffic tunnels) and/or where change-out costs are extremely high. The typical lifetime of conventional fixtures is about 20 years, corresponding to a light-producing device usage of at least about 44,000 hours (based on usage of 6 hours per day for 20 years). Light-producing device lifetime is typically much shorter, thus creating the need for periodic change-outs.
Accordingly, for these and other reasons, efforts have been ongoing to develop ways by which solid state light emitters can be used to a greater extent and/or in place of incandescent lights, fluorescent lights and other light-generating devices in a wide variety of applications. In addition, where light emitting diodes (or other solid state light emitters) are already being used, efforts are ongoing to provide lighting devices which are improved, e.g., with respect to energy efficiency, color rendering index (CRI Ra), contrast, efficacy (lm/W), and/or duration of service.
The most common type of general illumination is white light (or near white light), i.e., light that is close to the blackbody locus, e.g., within about 10 MacAdam ellipses of the blackbody locus on a 1931 CIE Chromaticity Diagram. The 1931 CIE Chromaticity Diagram (an international standard for primary colors established in 1931), and the 1976 CIE Chromaticity Diagram (similar to the 1931 Diagram but modified such that similar distances on the Diagram represent similar perceived differences in color) provide useful reference for defining colors as weighted sums of primary colors.
Because light that is perceived as white is necessarily a blend of light of two or more colors (or wavelengths), no single light emitting diode junction has been developed that can produce white light. “White” solid state light emitting lamps have been produced by providing devices that mix different colors of light, e.g., by using light emitting diodes that emit light of differing respective colors and/or by converting some or all of the light emitted from the light emitting diodes using luminescent material. For example, as is well known, some lamps use red, green and blue light emitting diodes to generate light that is perceived as white. Other lamps use (1) one or more light emitting diodes that generate blue light and (2) luminescent material (e.g., one or more phosphor materials) that emits yellow light in response to excitation by light emitted by the light emitting diode, whereby the blue light and the yellow light, when mixed, produce light that is perceived as white light. While there is a need for more efficient white lighting, there is in general a need for more efficient lighting in all hues.
Representative examples of the use of solid state light emitters to provide a lighting device that provides excellent energy efficiency and excellent CRI and excellent duration of service at a reasonable cost are described in U.S. Pat. No. 7,213,940, issued on May 8, 2007. Despite the existence of such devices, there will always be a desire to provide lighting devices which can be used in some situations and which can provide even better energy efficiency, even if such improvement in energy efficiency detracts some other aspect of the device to some degree.
In summary, therefore, there is an ongoing need for ways to use solid state light emitters, e.g., light emitting diodes, in a wider variety of applications, with greater energy efficiency, acceptable color rendering index (CRI), lower cost, and/or longer duration of service.